The present invention relates generally to an image-formation optical system and an imaging system that incorporates the same. More particularly, the invention is concerned with imaging systems such as digital still cameras, digital video cameras, small-format cameras mounted on cellular phones and personal computers, and surveillance cameras or the like, all harnessing solid-state image pickup devices, e.g., CCDs or CMOSs,
In recent years, electronic cameras using solid-state image pickup devices such as CCDs or CMOSs to take subject images have come into wide use in place of silver-halide film cameras. For imaging systems mounted on portable computers, cellular phones, etc. among those electronic cameras, size and weight reductions are especially demanded.
Some conventional image-formation optical systems used with such imaging systems are made up of one or two lenses. With those optical systems, however, any high performance is not expectable because of their inability to correct field curvature, as already known from discussions about aberrations. To achieve high performance, therefore, it is required to use three or more lenses.
Referring on the other hand to a CCD, as off-axis light beams leaving an image-formation lens system are incident on an image plane at too large an angle, a microlens fails to perform its own light-condensation capability, offering a problem that the brightness of an image changes extremely between the central and the peripheral portion of the image. Thus, the angle of incidence of light on the CCD, that is, the position of an exit pupil becomes important in view of design. To an optical system comprising fewer lenses, the position of an aperture stop becomes important.
Among image-formation lenses with these problems taken into account, there is a triplet type with a stop located at the front, as set forth typically in the following patent publications 1, 2, 3, 4, 5 and 6.
However, all those prior arts have problems in conjunction with performance and size.
Patent Publication 1
JP-A 1-144007
Patent Publication 2
JP-A 2-191907
Patent Publication 3
JP-A 4-153612
Patent Publication 4
JP-A 5-188284
Patent Publication 5
JP-A 9-288235
Patent Publication 6
JP-A 2001-75006